Arenas, stadiums, entertainment and sports facilities are ideally suited for dome coverings. Domed roof structures provide enhanced lighting, optimum seating visibility and satisfy a desire for a feeling of openness.
The early domed roof structures were steel-braced domes of varying designs which were capable of reaching a clear span of almost 700 feet. Steel-braced dome structures have the capacity to carry loads in both the radial and circumferential directions. The most noteworthy of the braced dome designs is the Superdome over the sports stadium in New Orleans. The advantages of the braced dome lie in its ability to resist loads with a force system acting in the surface of the shell in the radial and circumferential directions. The disadvantages of the steel-braced dome design lies in the fact that it is heavy, costly and difficult to construct. In addition single layer braced domes of large span, especially under unsymmetrical loads, may exhibit instability behavior in a snap - through buckling mode.
To achieve lower construction costs and to improve performance of dome structures, the cable-truss dome was designed. Various cable-truss designs have been disclosed in U.S. Pat. No. 4,736,553 issued to Geiger; U.S. Pat. No. 4,757,650 issued to Berger; and U.S. Pat. No. 5,259,158 issued to Levy. The cable-truss design is lighter, easier to construct and less expensive to build than the traditional steel-brace dome. Other advantages include the use of continuous tension members, such as cables, to form low shallow arches that support a lightweight membrane cladding which gives bracing to the truss.
However, the cable-truss design is limited by its inability to carry loads in the circumferential direction. In addition, the design of the cable-truss dome requires an outer compression ring. The stadium, therefore, exists separate from the compression ting which functions as an anchor for the cable net system. The need for a compression ring limits the cable-truss design's ability to cover structures having straight walls. Since the majority of members are tension only members, such as cables, a large pre-stress to the cable truss must be introduced to prevent the cables from going slack under applied loads and to also increase the overall stiffness of the cable-truss system. Under a down load the cable-truss dome will lose load in the top and increase load in the bottom, in order to prevent this load shifting the cables must be prestressed to a very high tension. The stresses which this places on other members of the system are balanced by the compression ting. The need for a compression ting results in the inability of the cable-tress dome to be readily adaptable to stadiums designed to have fiat walls. Moreover, the compression ting adds significant expense to the cost of constructing the cable-truss dome.
Part of the cable-truss design's appeal lies in the fact that it can be constructed by way of a pre-assembly on the ground instead of the piecemeal erection required with the braced-dome design. However, the use of continuous tension only members, such as cables, requires sophisticated computer analysis of each construction step in order to ensure overall building stability and intermittent cable forces, due to the large displacements the structure undergoes during the construction phase. Moreover, the preferred means for erecting the cable-truss dome is by building it in an inverted position as taught by Richard Buckminster Fuller in U.S. Pat. No. 3,139,957, a rather difficult and time consuming method of erecting the dome.
As a result, there continues to be a need in the industry for an economical roof structure which is capable of spanning large areas and flat wall designs and which provides increased stability by being able to carry compressive and tensile loads in both the radial and circumferential directions. In addition, there is a need for a roof structure which can be fined-tuned by adjusting the tension in the tension members to make it equally as efficient to carry compressive loads under a downward live load as it is to carry tension under an upward wind load. Likewise, there is a need for a roof structure which is easy to construct and allows construction to progress sequentially from the outer perimeter to the center without the need for shoring towers.